A class of amulets even older than the Egyptian scarabs is represented by the engraved Assyrio-Babylonian cylinders. There has been much discussion among scholars as to the original purpose for which these cylinders were made, some holding that they were exclusively employed as seals or signets, while others incline to the belief that many of them were intended only for use as amulets or talismans. These cylinders are perforated and were worn suspended from the neck or wrist, as is most frequently the case with talismans, and the engraved designs often represent religious or mythological subjects, the accompanying inscription merely consisting of the names of the gods. Cylinders of this type could not have been used as personal signets, and it is quite possible that Dr. Wiedemann is right in supposing that their imprint on a document was considered to impart a certain mystic sanction to the agreement, and render the divinities or spirits accountable for the fulfilment of the contract.[178] The oldest known form of seal is the cylinder. Babylonian and Assyrian cylinder-seals are known of a date as early as 4000 B.C. From the earliest period until 2500 B.C. they were made of black or green serpentine, conglomerate, diorite, and frequently of the central core of a large conch shell from the Persian Gulf. From 2500 B.C. to 500 B.C. the cylindrical form was prevalent, and the materials include a brick-red ferruginous quartz, red hematite (an iron ore), and chalcedony, a beautiful variety of the last-named stone known as sapphirine being sometimes used. On the cylinders produced from 4000 B.C. to 2500 B.C. the designs most frequently represent animal forms; on those dating from 2500 B.C. to 500 B.C. are generally inscribed five or six rows of cuneiform characters. Up to the last-named date the work was all done by the sapphire point, and not by the wheel, and it is not until the fifth century B.C. that wheel work is apparent in any Babylonian or Assyrian stone-engraving. In the course of the sixth century B.C. the cylindrical seals became less frequent, and the tall cone-like seals came into use.[179] A new type makes its appearance about the fifth or sixth century B.C., namely, the scaraboid seal introduced from Egypt. From the third century B.C. until the second or third century A.D., the seals became lower and flatter, and the perforation larger, until they sometimes assumed the form of rings; later the ring form becomes general. They are usually hollowed a little in the middle, which gives them the shape and size of the lower short joints of a reed; indeed, it has been suggested that the original seal was rudely patterned after a reed joint. The materials used for these cylinders include lapis-lazuli, very freely used and probably from the Persian mines, jasper, rock-crystals, chalcedony, carnelian, agate, jade, etc.; a hard, black variety of serpentine is perhaps the most common of all the materials used for this purpose.[180]